The sentence is made of two clauses "When her mother called her" and "Lisa went with her to the car". Each clause has a subject (a noun that performs an action) and a verb (the action the subject performs). In the second clause, "Lisa" is the subject, who performs an action (verb) "went". A word that modifies the action is required. Usually a word modifies an action in terms of manner and the word is an adverb. Adverbs modify verbs, adjectives and other adverbs. From the given options, 'readily' is an adverb that refers to the manner of the action "went". Hence option C is correct. 'Ready' is an adjective that means 'prepared for an action or situation, in a suitable state'. An adjective modifies a noun by describing it, it does not modify a verb. Hence option A is incorrect. Similarly, 'quick' is also an adjective, it means 'fast' or 'prompt' and modifies a noun, adjectives don't modify verbs, hence option D is incorrect. 'Readier' is the comparative form of the adjective 'ready' used to compare the degree of being ready between two individuals. The sentence is not a comparison, hence option B is incorrect.